Segment and Related Information - Financial Data for Business Segments (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($) state	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) state	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Segment Reporting Information [Line Items]
Number of States in which Entity Operates | state	4		4
Operating revenues	$ 5,401	$ 5,339	$ 13,921	$ 14,450
Segment net income (loss)	959	718	2,096	1,680
Total assets	75,215		75,215		$ 70,923
Kemper IGCC [Member]
Segment Reporting Information [Line Items]
Pre-Tax Charge To Income	150	418	182	798	868	$ 1,200
After Tax Charge To Income	93	258	112	493	536	$ 729
Electric Utilities [Member]
Segment Reporting Information [Line Items]
Operating revenues	5,390	5,327	13,887	14,408
Segment net income (loss)	976	722	2,093	1,685
Total assets	74,386		74,386		70,063
Intersegment Eliminations [Member] | Electric Utilities [Member]
Segment Reporting Information [Line Items]
Operating revenues	(109)	(115)	(322)	(301)
Segment net income (loss)	0	0	0	0
Total assets	(404)		(404)		(131)
Consolidation Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	(26)	(22)	(86)	(72)
Segment net income (loss)	1	(2)	0	(5)
Total assets	(651)		(651)		(296)
Traditional Operating Companies [Member] | Electric Utilities [Member]
Segment Reporting Information [Line Items]
Operating revenues	5,098	5,007	13,123	13,594
Segment net income (loss)	874	658	1,912	1,557
Total assets	67,750		67,750		64,644
Southern Power [Member]
Segment Reporting Information [Line Items]
Operating revenues	104	103	303	243
Southern Power [Member] | Electric Utilities [Member]
Segment Reporting Information [Line Items]
Operating revenues	401	435	1,086	1,115
Segment net income (loss)	102	64	181	128
Total assets	7,040		7,040		5,550
All Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	37	34	120	114
Segment net income (loss)	(18)	$ (2)	3	$ 0
Total assets	$ 1,480		$ 1,480		$ 1,156